TN 

5Z5 

Ya 


IRLF 


77    fiSfi 


OlFT 

v^-*--*-   -*• 

UNIVERSITY  OFJNEVADA  BULLETIN 


VOL.  V  NOVEMBER  15,  1911  No.  6 


SLIME-FILTRATION 


BY 


GEORGE    3.  \YOTJNG 

Mack  ay  School  of  Mines 


A  Paper  presented  to  the  American  Institute  of  Mining  Engineers 
at  the  San  Francisco  Meeting,  October,  1911,  and,  by  Permission 
of  the  Council,  reprinted  from  the  BULLETIN  of  the  American 
Institute  of  Mining  Engineers,  No.  59,  November,  1911 


PUBLISHED  QUAETEELY  BY  THE  UNIVEESITY  or  NEVADA 
RENO,  NEVADA 

Entered  in  the  Postoftice  at  Reno,  Nevada,  as  second-class  matter  under  the  Act  of  Congress 

July  16,  1894 


NOTE. — The  University  of  Nevada  publications 
offered  in  exchange  for  certain  periodicals  and  for  the 
publications  of  learned  societies  and  institutions,  uni- 
versities, and  libraries.  For  sample  copies  address  the 
University  Library,  Reno,  Nevada. 

JAMES  EDWARD  CHURCH,  JR., 
CARL  ALFRED  JACOBSON, 
HERBERT  WYNFORD  HILL, 

Committee  on  Publications. 


SUBJECT   TO   REVISION. 

[TRANSACTIONS  OF  THE  AMERICAN  INSTITUTE  OF  MINING  ENGINEERS.] 


Slime-Filtration. 

BY  GEORGE  J.    YOUNG,*  RENO, /N 
(San  Francisco  Meeting,  October,  1911.) 

THE  nature  of  slimes  handled  in  the  treatment  of  gold-  and 
silver-ores  has  been  discussed  in  technical  literature  to  a  con- 
siderable extent.  The  subject  of  slime-filtration  from  the  practi- 
cal worker's  stand-point  has  also  received  much  comment,  and 
scattered  through  the  literature  of  the  subject  are  descriptions 
of  many  slime-filtration  installations.  Articles  of  this  nature 
serve  a  valuable  purpose  and  assist  materially  in  the  design  of 
new  and  the  improvement  of  old  plants.  The  subject  of  the 
physics  of  slime-filtration  has  been  touched  upon  to  only  a 
slight  extent.  The  underlying  principles  are  worthy  of  more 
intensive  study  and  experimentation  than  they  have  received, 
and  the  main  purpose  of  this  paper  is  to  present  the  results  of 
such  study  and  experimental  work  as  will  serve  to  make  clear 
in  part  at  least  many  of  the  principles  which  control  the  filtra- 
tion of  slime. 

Nature  of  Slime. 

Much  has  been  written  concerning  an  accurate  definition  of 
the  term  "  slime,"  but  no  comprehensive  definition  seems  to  be 
generally  accepted.  The  reason  for  this  is  clear.  A  slime 
consists  of  at  least  three  different  substances,  each,  when  sepa- 
rated, possessing  distinctly  different  physical  and  to  a  certain 
extent  chemical  properties.  These  substances  are  extremely 
fine  sand,  a  colloidal  material  which  may  be  and  generally  is 
in  a  coagulated  condition,  and  a  colloidal  material  which  is  in 
a  non-coagulated  condition.  Suspending  a  slime  in  a  relatively 
large  volume  of  water  by  shaking  and  allowing  sedimentation 
to  take  place,  results  in  the  fine  sand  settling  out  with  com- 
parative rapidity,  followed  by  the  coagulated  material,  which 
settles  much  more  slowly  and  finally  a  certain  portion  remains 

*  Professor  of  Mining  and  Metallurgy,  Mackay  School  of  Mines. 

en 
515 


840  SLIME-FILTRATION. 

indefinitely  suspended.  To  the  settled,  coagulated  portion 
some  writers  have  given  the  terra  gel,  and  to  the  suspended 
portion  the  term  sol.  The  physical  properties  which  distinguish 
the  fine  sands  are,  the  angular  character  of  the  grains  and  the 
comparatively  rapid  settling  in  water.  With  most  quartzose 
ores  the  sand  grains  are  composed  of  silica,  although  con- 
stituents of  the  ore,  such  as  silicates  or  oxides,  also  characterize 
the  sand  portion  of  the  slime.  The  coagulated  colloid  consists 
of  aggregates  of  rounded  grains  together  with  individual 
grains,  settles  much  less  rapidly,  possesses  the  property  of  floc- 
culation  and  deflocculation,  has  the  property  of  absorbing  cer- 
tain dyes,  and  a  distinctive  chemical  composition.  Clays  and 
hydrated  silica  are  the  two  colloids  most  likely  to  occur  in 
quartzose  ore.  The  former  is  a  common  constituent  of  many 
ores,  the  latter  is  perhaps  seldom  present.  For  practical  pur- 
poses clay,  or  hydrated  aluminum  silicate,  may  be  considered  to 
be  the  chief  colloidal  constituent,  and,  mixed  with  fine  sand, 
to  constitute  the  coagulated  portion  of  a  slime.  Inasmuch  as 
the  ordinary  mill-slime  is  quite  well  coagulated  by  the  liberal 
use  of  lime,  the  metallurgist  has  to  deal  only  with  mixtures  of 
fine  sands  and  coagulated  colloid. 

The  distinctive  properties  of  a  slime  depend  upon  the  rela- 
tive proportions  of  fine  sand  and  colloid.  Assuming  all  colloid 
and  no  fine  sand,  we  would  have  a  material  which  could  not  be 
leached,  and  which  would  filter  very  slowly  and  under  certain 
conditions  not  at  all;  assuming  all  fine  sand  and  no  colloid, 
we  would  have  a  leachable  material.  In  a  moist  condition  a 
slime  may  be  likened  to  a  clay;  with  a  large  proportion  of 
sand  a  "  short  clay  "  or  a  clay  of  moderate  plasticity  would  be 
the  result ;  with  a  small  proportion  of  sand  a  "  fat "  clay  or  a 
clay  with  a  high  degree  of  plasticity  would  result.  With  suf- 
ficient moisture  a  slime  partakes  of  the  character  of  a  viscous 
fluid,  and  in  this  very  fine  sand  will  be  almost  indefinitely 
suspended,  and  little  or  no  separation  of  fine  sand  from  colloid 
will  result.  This  latter  statement  is  true  of  sand  finer  than  a 
150-mesh  screen.  With  coarser  sand,  the  coarse  sand  particles 
tend  to  settle  out  quite  rapidly.  By  increasing  the  proportion 
of  water  successive  crops  of  finer  and  finer  sands  can  be  settled 
out  until  a  point  is  reached  where  the  particles  of  coagulated 
colloid  and  the  finest  sands  settle  at  the  same  rate.  Beyond 

[2] 


SLIME-FILTRATION.  841 

this  point  no  further  separation  of  sand  from  colloid  is  possible. 
No  sharp  line  in  the  mechanical  separation  of  sand  from  col- 
loidal material  being  possible,  it  is  necessary  to  use  a  definition 
which  will  embody  some  limitation  as  to  the  size  of  the  maxi- 
mum sand  grain.  Successive  screen-sizes  have  been  used ; 
first  a  100-mesh  screen,  then  a  150-mesh  screen,  and,  finally,  a 
200-mesh  screen ;  and  this  is  the  accepted  present  practice  in 
milling-work.  All  material  in  a  pulp  finer  than  a  200-mesh 
screen  is  considered  as  slime.  The  definition,  that  a  slime  is 
the  unleachable  portion  of  a  mill-pulp,  is  still  in  use.  . 

A  more  comprehensive  definition  than  the  foregoing  is :  a 
slime  consists  of  a  mixture  of  sands  finer  than  150-  or  200- 
mesh  screen  with  an  amorphous  clay-like  material,  consisting 
principally  of  hydrated  aluminum  silicate. 

The  general  method  of  slime-treatment  is  to  agitate  the  slime 
with  a  cyanide  solution  for  a  sufficient  time  to  dissolve  the 
gold,  and  then,  either  to  filter  off  the  surplus  solution  and  dis- 
place the  remainder  with  water,  or  to  thicken  the  slime  by 
settlement  and  decantation,  and  then  to  filter  and  displace  the 
remaining  solution  by  water. 

The  mechanical  appliances  in  use  for  filtration  are  grouped 
as  follows : 

I.  Suction-filters,  or  filters  in  which  a  vacuum  is  used  to  accelerate  filtra- 
tion. 

A.  Appliances  using  a  thin  slime-cake  and  practically  continuous  in  their  action. 

(Oliver  and  Ridgway  filters.) 

B.  Appliances  using  a  thick  slime-cake  and  intermittent  in  their  action.    (Moore 

and  Butters  filters. ) 

II.  Pressure-filters,  or  filters  in  which  hydrostatic  head,  compressed  air  or 
pumps  are  used  in  order  to  secure  greater  pressures  than  are 
possible  with  a  vacuum-pump. 
These  filters  are  intermittent  in  their  action. 

C.  Ordinary  filter-presses. 

D.  Sluicing  filter-presses  (Merrill  filter-press). 

E.  Filtering-chambers  or  cylinders ;   filters   in  which  the  filtering-basket  is  in- 

closed in  a  cylinder.     (Burt,  Kelley,  and  Sweetland  filter-presses. ) 
III.  Centrifugal  filters,  or  filters  in  which  centrifugal  force  is  used  to  sepa- 
rate solution  from  slime. 
These  filters  are  continuous  in  action. 

The  filters  in  Sections  I.  and  II.,  with  the  exception  of  the 
Ridgway,  employ  vertical  filtering-surfaces.  The  Oliver 1  makes 

»  Trans.,  xli,  349  to  356  (1911). 

[3] 


842  SLIME-FILTRATION. 

use  of  a  revolving  cylindrical  surface  as  a  filtering-surface. 
Centrifugal  filters  are  in  process  of  development,  and  have  not 
as  yet  secured  any  foothold  in  gold-  and  silver-metallurgy.  It 
is  not  improbable,  however,  that  some  comparatively  simple 
filter  based  on  the  use  of  centrifugal  force  will  be  perfected, 
and  will  successfully  compete  with  the  other  forms.  At 
present  the  suction-filters  are  in  greatest  use.  Of  the  pressure- 
filters,  the  ordinary  filter-presses  have  gone  out  of  use,  except 
as  clarifying-presses,  and  filters  of  groups  D  and  E  only  are 
in  use. 

The  development  of  slime-filtration  is  of  interest.  Filter- 
presses  and  filtering-beds  in  vats  were  first  used.  The  filtering- 
beds  were  soon  discarded  and  the  filter-press  systematically 
developed.  The  size  of  the  press  was  increased,  mechanical 
devices  to  facilitate  discharge  and  decrease  the  proportion  of 
labor  required  were  invented  and  introduced ;  but  in  spite  of 
all  this  the  cost  of  treatment  in  filter-presses  remained  high. 
In  western  America  the  filter-press  never  received  much  recog- 
nition, but  in  Australia  filter-pressing  was  extensively  intro- 
duced, and  slime  was  successfully  handled  by  this  method.  It 
remained  for  an  American,  Charles  A.  Merrill,  to  complete  the 
last  improvement  in  the  filter-press.  By  the  introduction  ot 
the  sluicing-system  the  slime-cakes  could  be  washed  out  of  the 
filter-cells  and  the  press  operated  without  opening  or  separat- 
ing the  filter-plates  for  each  charge.  This  improvement 
reduced  the  labor  and  cost  and  increased  the  effectiveness  of 
the  filter-press.  The  Merrill  press  represents  the  culminating 
point  in  the  filter-press  line  of  development  in  slime-filtration. 

The  Moore  filter  was  the  first  suction-filter  in  the  field,  and, 
while  it  did  not  score  any  very  decided  success  in  the  first  in- 
stallations, it  did  attract  the  attention  of  metallurgists  to  the 
idea  involved.  "While  the  Moore  Filter  Co.  was  perfecting  the 
mechanical  features  of  its  filter,  the  Batters  filter  was  intro- 
duced, and  so  many  of  the  difficulties  of  the  Moore  filter  were 
overcome  in  the  Butters,  that  this  latter  filter  received  wide- 
spread recognition  and  was  introduced  into  many  milling- 
plants.  The  Moore  filter  introduced  the  idea  of  the  canvas- 
covered  filtering-cell  immersed  in  the  slime-pulp  and  utilizing 
suction  to  draw  the  solution  through  the  walls  of  the  cell  and 
to  build  up  a  cake.  The  necessary  transfers  are  made  by  lift- 

[4] 


SLIME-FILTRATION.  843 

ing  the  filtering-basket  out  of  the  pulp.  The  Butters  filter 
introduced  the  idea  of  a  stationary  filtering-cell,  and  effected 
the  transfers  by  pumping  the  slime-pulp  and  wash-water  from 
the  vat  in  which  the  filtering-cells  were  immersed.  The  rela- 
tive merits  of  the  two  systems  have  been  sufficiently  discussed 
in  the  technical  literature.  Both  the  Moore  and  the  Butters 
filter  have  reached  a  point  where  little  or  no  further  improve- 
ment seems  possible.  Like  the  Merrill,  either  one  of  these 
systems  will  satisfactorily  meet  the  requirements  of  slime- 
filtration. 

The  combination  of  the  ideas  involved  in  the  filter-press 
and  the  suction-filter  is  seen  in  group  E,  or  the  filtering- 
chambers.  The  Kelley,  the  Burt,  and  the  Sweetland  may  be 
compared  to  a  Butters  filter  installed  in  a  pressure-tank. 

The  effort  to  secure  a  continuously-acting  filter  has  resulted 
in  two  important  types  being  developed,  of  which  the  Ridgway 
and  the  Oliver  are  the  best  known.  Both  of  these  filters  utilize 
a  comparatively  thin  slime-cake.  Both  operate  very  success- 
fully, and  compared  with  the  thick-cake  machines  have  de- 
cided advantages,  briefly  stated  as  :  simplicity  of  design ;  prob- 
ably lower  capitalization-charges  for  equal  capacities;  lower 
operating-costs;  and  less  attention  required  in  the  operation. 

With  the  exception  of  the  Oliver  filter,  the  general  method 
of  operation  of  both  suction-  and  pressure-filters  is  the  same. 
The  slime-pulp  is  delivered  to  the  filter  in  the  proportion  of 
one  of  dry  slime  to  from  three  to  one  of  solution.  The  pulp 
is  forced  into  the  cells  of  the  pressure-filters  and  a  cake  formed 
against  the  canvas  walls  of  the  cells,  the  surplus  pulp,  if  any, 
is  withdrawn,  and  wash- water  forced  in  until  the  contained  solu- 
tions are  displaced.  The  cake  is  then  forced  off  from  the  can- 
vas surface,  either  by  water  or  air  or  a  combination  of  both, 
and  sluiced  out.  In  the  vacuum-filter  the  filtering-cells  are 
immersed  in  the  pulp,  a  vacuum  is  formed,  and  a  cake  built  up; 
the  surplus  pulp  is  then  withdrawn  either  by  lifting  the  filtering- 
cells  out  or  by  withdrawing  the  pulp  by  pumps,  and  the  cakes 
are  immersed  in  water  for  washing.  In  the  Moore  filter  the 
cakes  are  discharged  by  forcing  them  off  from  the  cell  by  water 
or  air  and  dropping  into  a  hopper  for  sluicing  away;  in  the 
Butters  the  cake  is  forced  off  in  the  same  way,  but  while  still 
immersed  in  the  wash-solution.  The  wash-solution  is  then 

[5] 


844 


SLIME-FILTRATION. 


withdrawn,  either  by  decanting  or  pumping,  and  the  slime-cake 
and  surplus  wash  sluiced  out.  The  Oliver  filter  performs  the 
operations  of  cake-formation,  washing,  and  discharge  in  con- 
tinuous sequence.  Three  steps  may  be  designated  as  common 
to  all  these  filters :  cake-formation,  washing,  and  discharge. 
The  cycle  of  operations  of  the  more  common  forms  of  filters 
is  shown  in  Fig.  1.  Typical  examples  have  been  taken  in  each 
case. 

The  conditions  under  which  slime-cakes  are  formed  and 
washed  are  the  critical  points  to  be  considered ;  the  discharge 
and  sluicing  away  of  the  cake  is  a  comparatively  simple  mat- 


OLIVER  FILTER 
4-m.  Cycle 


RIDGWAY  FILTER 
1-m.  Cycle 


BUTTERS  FILTER 
180-m.  Cycle 


MOORE  FILTER 
145-m.  Cycle 


MERRILL  FILTER 
215  in.  Cycle 


BURT  FILTER 
62-in.  Cycle 


FIG.  1. — CYCLE  OF  OPERATION  OF  VARIOUS  FILTERS. 

ter  and  requires  no  special  comment.  My  experimental  work 
was  largely  confined  to  suction-filtration,  and  pressure-filtration 
was  only  briefly  studied.  The  method  of  carrying  out  the  ex- 
periments may  be  summarized  as  follows :  After  trying  out 
several  different  sizes  and  types  of  filter-cells  a  test-filter  of  0.5 
sq.  ft.  filtering-surface  was  decided  upon,  shown  in  Fig.  2.  A 
ribbed  wooden  support  with  J-in.  grooves  and  J-in.  ribs  was 
used  to  support  the  canvas  surface.  Brass  side-strips  and  a 

[6] 


SLIME-FILTRATION. 


845 


slotted  brass  bottom-strip  were  used  to  protect  the  cake  and 
to  assist  in  measuring.  A  type  slime  was  obtained  by  classify- 
ing a  pulp  from  a  Tonopah  quartzose  ore  which  had  been 
crushed  in  a  stamp-battery.  The  slime  was  settled  by  the  use 
of  lime,  and  then  by  repeated  settlement  all  the  coarse  and  as 
much  of  the  fine  sand  as  possible  were  settled  out  and  removed. 


END 


FIG.  2.— FILTER  USED  IN  EXPERIMENTAL  WORK. 

The  slime-pulp  remaining  was  settled  to  a  thickness  giving  a 
density  of  1.3.    The  screen-analysis  of  this  slime  approximated : 


On  100-mesh, 

Plus  150,  minus  100,        . 

Plus  200,  minus  150,        . 

Minus  200,  and  less  than  2  min.  settling, 

Settling  in  from  2  to  4  min.,    . 

Settling  in  from  4  to  8  min.,  . 

Remainder,     ...... 


Per  Cent. 
0.1 
1.8 

1.20 

20.0 

8.5 

11.6 

54.3 


846  SLIME-FILTRATION. 

Of  this  slime-pulp,  97.4  per  cent,  passed  a  200-mesh  screen. 
Fine  sand  passing  a  100-mesh  screen  was  used  in  securing  the 
necessary  mixtures.  The  nitrate  was  measured  in  a  Woulff 
bottle,  to  which  was  attached  a  vacuum-gauge.  The  vacuum 
was  obtained  by  a  small  single-acting  pump  exhausting  from  a 
10-gal.  vacuum-tank.  A  short  length  of  hose  connected  the 
Woulff  bottle  with  the  tank.  The  slime-mixtures  were  made 
up  in  buckets  and  heated  to  the  temperatures  as  required. 
Variations  in  pressure,  temperature,  and  slime  were  the  main 
points  studied. 

Filtering-Rate. 

The  rapidity  with  which  a  cake  may  be  formed  depends  upon 
the  filtering-rate  of  the  slime,  the  thickness  of  the  cake,  the 
temperature  and  density  of  the  pulp,  and  the  intensity  of  the 
vacuum.  The  filtering-rate  of  a  slime,  which  is  numerically 
defined  in  this  paper  as  the  number  of  pounds  of  water  drawn 
through  100  sq.  ft.  of  filtering-surface  per  minute,  depends,  for 
a  cake  of  given  thickness,  upon  the  character  of  the  slime,  the 
density  of  the  slime-cake,  the  suction-pressure,  the  temperature, 
and,  to  a  moderate  extent,  upon  the  character  of  the  filtering- 
surface  and  its  support.  These  factors  are  so  interrelated  that 
it  is  impossible  to  conduct  any  series  of  experiments  which 
would  exactly  show  the  effect  of  varying  them.  At  best  the 
results  are  approximations. 

Fig.  3  shows  the  variation  of  the  filtering-rate  with  variable 
thickness  of  slime-cake  both  while  building  up  and  in  clear 
water.  The  curves  represent  the  averages  of  a  number  of  tests 
in  which  temperature  and  pressure  were  practically  the  same 
for  all.  A  No.  10  canvas  was  used.  In  carrying  out  the  ex- 
periment the  filter  was  immersed  in  the  pulp  for  5  or  10  min. 
and  a  cake  built  up.  This  cake  was  then  quickly  removed,  its 
thickness  measured,  and  the  filter  immersed  in  clear  water. 
After  determining  the  filtering-rate,  the  filter  was  replaced  in  the 
pulp  and  an  additional  thickness  built  up.  The  filtering-rate 
during  building  up  was  determined  by  calculation  from  the 
amount  of  water  passing  while  building  to  a  given  thickness. 
The  difference  between  the  two  curves  is  comparatively  slight 
and  indicates  that  the  filtering-rate  during  building  up  a  cake 
is  greater  in  the  pulp  than  in  clear  water  for  thin  cakes,  while 
for  the  thicker  cakes  the  reverse  is  true. 

[8] 


SLIME-FILTRATION. 


847 


Fig.  4  shows  the  effect  of  variation  of  pressure  upon  the  fil- 
tering-rates of  cakes  of  varying  thickness.  Three  pressures 
were  used — 11.35,  17,  and  21.5  in.  of  mercury.  The  last 
pressure  is  about  the  maximum  obtainable  in  Nevada  practice. 
The  general  effect  of  increase  of  pressure  is  to  increase  the  fil- 
tering-rate. This  is  more  marked  with  the  thin  cakes,  while 
with  the  thick  cakes  all  three  curves  tend  to  run  together. 
With  thick  cakes  the  effect  of  an  increase  of  pressure  is  to  in- 
crease the  density  of  the  cake  and  thus  reduce  its  permeability. 
With  higher  pressures  this  effect  is  more  marked,  and  indi- 


1.  During  building  up  cake 

2.  After  building  up  cake  and  in  clear  water 


100 

90 

80 

h-' 
u_ 

g70 

8 

-60 

UJ 

CL 

£C  50 
111 

h 

£40 

u_ 

O 

230 

z 
:D 

20 


0.1       0.2       0.3       0.4       0.5       0.6       0.7      0.8       0.9       1.0       1.1       1.2 
THICKNESS  IN  INCHES 

FIG.  3. — AVERAGE  FILTERING-RATES  FOR  SLIME-CAKES,  No.  10  CANVAS. 

cates  that  a  point  would  soon  be  reached  where  the  increased 
pressure  would  result  in  decreased  filtering-rate.  This  is  par- 
ticularly true  of  slirne  containing  a  small  proportion  of  sand,  and 
much  less  so  with  slimes  containing  a  large  proportion  of  sand. 
Sweetland,  in  his  paper,  Pressure  Filtration,2  shows  for  pres- 
sures up  to  65  Ib.  per  sq.  in.  a  progressive  increase  in  the 
filtering-rate  for  slime-cakes  varying  from  0.5  to  1.75  in.  The 
slime  used  in  the  Sweetland  experiments  was  obtained  from  the 
Goldfield  Consolidated  mill.  Unfortunately,  neither  a  physical 
analysis  of  the  slime  nor  the  density  of  the  slime-cakes  formed 

2  Mining  and  Scientific  Press,  vol.  xcix.,  No.  26,  p.  853  (Dec.  25,  1909). 

[9] 


848 


SLIME-FILTRATION. 


is  given  in  the  paper.  The  slime-pulp  of  the  Goldfield  Con- 
solidated mill  is  distinctly  of  a  sandy  nature  and  would  be 
expected  to  give  results  of  this  kind,  whereas  a  very  clayey 
pulp  would  give  results  of  an  opposite  character.  Experiments 
with  a  slime  similar  to  the  type  slime,  and  with  pressures  rang- 
ing from  10  to  30  Ib.  per  sq.  in.,  showed  an  increase  in  filtering- 
rate  from  11  to  16  Ib.  of  water  per  100  sq.  ft.  per  min.  for  a 
cake  of  0.25  in.  thick;  for  a  0.5-in.  cake  an  increase  in  pres- 
sure from  20  to  30  Ib.  decreased  the  filtering-rate  from  10  to  7 
Ib. ;  for  a  0.75-in.  cake  an  increase  in  the  filtering-pressure 
from  20  to  30  Ib.  made  no  difference  in  a  filtering-rate  of  6  Ib. 
R.  Oilman  Brown,  in  his  paper,  Cyanide  Practice  with  the 
Moore  Filter,3  in  discussing  the  treatment  of  a  very  clayey 


IN 

g 


§20 
5 

SID 


A.    17  inches  suotiou-cake  contains  35.4.per  cent,  water 
B-    11.35  inches  suctiou-cake  contains  36.75  per  cent,  water 
C.     21.5  inches  suction-cake  contains  35.12  per  cent,  water 


0.3      0.4      0.5       0.6      0.7 
THICKNESS  IN  INCHES 


0.8       0.9       1.0 


FIG.  4. — EFFECT  OF  VARIATION  OF  PRESSURE  UPON  FILTERING-KATES, 
No.  10  CANVAS. 

slime  at  Bodie,  says :  "  Filter-pressing  was  tried  and  aban- 
doned, because  an  eighth  of  an  inch  of  pure  slime  would  make 
the  cloths  impervious,  even  under  120-lb.  pressure ;  and  even 
if  the  slime  was  mixed  with  fine  sand,  the  filtering  was  so  slow 
that  the  sand  settled  out  in  the  chambers,  with  the  same  re- 
sult." The  practical  conclusion  that  may  be  drawn  from  a 
study  of  the  effects  of  pressure  in  filtration  is  that,  with  mate- 
rial of  a  permeable  nature  such  as  a  sandy  slime,  increased 
pressures  over  those  obtainable  by  means  of  vacuum-pumps  are 
advantageous,  while  with  material  in  which  only  a  moderate  to 
a  small  amount  of  sand  is  present  and  the  permeability  low, 
the  use  of  higher  pressures  offers  no  advantages  over  those  ob- 
tainable by  vacuum-pumps.  In  the  use  of  both  the  Moore  and 


8  Mining  and  Scientific  Press,  vol.  xciii.,  No.  9,  p.  261  (Sept.  1,  1906). 

[10] 


SLIME-FILTRATION. 


849 


the  Butters  systems,  experiments  should  be  made  with  different 
intensities  of  vacuum,  for  it  may  be  found  that  a  vacuum  lower 
than  the  maximum  obtainable  with  the  available  apparatus 
will  give  a  higher  filtration-rate,  and  thus  decrease  the  time  for 
both  building  up  and  washing. 

Fig.  5  gives  the  comparative  filtering-rates  of  five  slimes. 
The  game  test-filter,  temperatures,  and  pressures  were  used  in 
each  case.  No.  10  canvas  was  used  on  the  filter.  The  slimes 
used  were :  a  clay  slime  (a  very  plastic  fire-clay)  containing 
about  40  per  cent,  of  sand  which  settled  out  in  1  min. ;  the 
average  of  the  results  on  the  type  slime ;  a  slime  from  a  Vir- 
ginia City  tailings-pond;  the  type  slime  containing  37  per 


0.1       0.2      0.3       0.4       0.5       0.6      0.7       0.8      0.9      1.0       1.1      1.2 
THICKNESS  IN  INCHES 

FIG.  5. — FILTERING-RATES  OF  FIVE  SLIMES,  No.  10  CANVAS. 

cent,  of  fine  sand  (determined  on  the  basis  of  1  min.  settle- 
ment); the  type  slime  with  52  per  cent,  of  fine  sand  (deter- 
mined on  the  basis  of  1  min.  settlement).  The  type  slime  on 
the  basis  of  1-min.  settlement  gave  6.5  per  cent,  of  fine  sand. 
B  and  C  respectively  represent  the  37  and  the  52  per  cent, 
of  fine-sand  slimes. 

The  filtering-rate  curves  for  the  type  slime  and  the  Virginia 
City  slime  are  coincident.  The  increase  in  the  proportion  of 
fine  sand  from  6.5  to  37  per  cent,  makes  but  very  little  differ- 
ence in  the  filtration-rate.  A  further  increase  to  52  per  cent, 
shows  a  marked  increase  in  the  filtering-rate  (curve  C).  While 
the  clay  slime  has  a  greater  proportion  of  fine  sand  than  either 

[ii] 


850 


SLIME-FILTRATION. 


the  type  or  jB,  the  filtering-rate  curve  is  much  lower.  The  con- 
clusions which  may  be  drawn  from  these  experiments  are : 
slimes  from  similar  ores  subjected  to  the  same  metallurgical 
treatment  give  similar  filtering-rate  curves;  a  moderate  varia- 
tion in  the  proportion  of  fine  sand  gives  filtering-rates  differing 


0.1       0.2       0.:?       0.4       0.5       0.6       0.7       0.8       0.9       1.0       1.1       1.2 

THICKNESS  OF  CAKE,  INCHES 

FIG.  6. — EFFECT  OF  TEMPERATURE  UPON  FILTERING- RATES  OF  SLIME- 
CAKES,  No.  12  DUCK. 


0.1       0.2       0.3       0.4       0.5       0.0       0.7       0.8       O.V)       1.0       L.I       l.<i       1.3 

THICKNESS  OF  CAKE,  INCHES 

FIG.  7. — EFFECT  OF  TEMPERATURE  UPON  FILTERING-KATES  OF  SAUD-  AND 
SLIME-CAKES,  No.  12  DUCK. 

only  to  a  small  degree,  while  a  considerable  increase  in  the 
proportion  of  fine  sand  increases  the  filtering-rate ;  the  propor- 
tion of  colloidal  matter,  or,  in  this  case,  clay  base,  has  a  marked 
influence  upon  the  filtering-rate;  much  more,  relatively,  than 

[12] 


SLIME-FILTRATION. 


851 


the  effect  of  fine  sands  in  increasing  the  filtering-rates.  The 
amount  of  clay  is  the  dominating  factor  in  filtering-rates,  and 
this  fact  is  indicated  by  the  curves  approaching  a  common 
point  as  the  thickness  of  the  cake  is  increased. 

Fig.  6  shows  the  effect  of  temperature  upon  the  filtering-rate 
of  the  type  slime.  A  No.  12  canvas  was  used  on  the  filter  for 
these  experiments.  For  thin  cakes  the  increase  in  filtering-rate 
is  more  marked  than  for  the  thicker  cakes.  The  same  ten- 
dency of  the  rate-curves  to  run  together  for  the  thicker  cakes 
is  to  be  noted. 


Clay  slime 
Average-sliine 

B.  37  per  cent,  sand 

C.  52  per  cent,  sand 

O.  sand  and  slime  15JC 
P.  sand  and  slime  24°3  C 
Q.sand  and^slime  33°2  C 


0.1       0.2       0.3       0.4      0.5       0.6      0.7       0.8      0.9       1.0       1.1       1.2       1.3 
THICKNESS  IN  INCHES 

FIG.  8.— COMPARISON  OF  FILTERING-RATES,  No.  12  CANVAS. 

Fig.  7  shows  the  effect  of  temperature  upon  a  slime  contain, 
ing  50  per  cent,  of  fine  sand  and  the  type  slime.  The  marked 
increase  in  filtering-rate,  with  moderately  elevated  tempera- 
tures, is  so  noticeable  as  to  indicate  a  condition  of  considerable 
practical  importance.  By  increasing  the  temperature  of  the 
pulp  greater  capacity  could  be  readily  obtained  with  a  given 
unit.  Fig.  8  compares  the  filtration-rates  of  the  clay  slime,  the 
type  slime,  and  the  several  sand-slime  cakes.  Fig.  9  compares 
the  filtering-rates  for  fine-sand  beds  3  in.  thick  under  varying 

pressures. 

Filtering-Surfaces. 

Most  of  the  suction-filters  employ  No.  10  canvas  duck  for 
the  filtering-surface.  The  Oliver  filter  makes  use  of  a  No.  12 

[13] 


852 


SLIME-FILTRATION. 


and  the  Merrill  filter-press  of  a  No.  6  duck  over  a  light  twill. 
In  the  Butters  and  the  Moore  filters  three  methods  of  support  are 
in  common  use.  The  original  Butters  unit  consisted  of  canvas 
stitched  at  close  intervals  over  a  center  sheet  of  cocoa  matting, 
which  gives  a  very  porous  gathering-space  for  the  solutions  and 
also  sufficient  support  to  the  canvas.  The  objections  to  this 
construction  are  the  cost,  and  the  clogging  of  the  matting. 
With  the  exception  of  the  Goldfield  Consolidated  mill,  all  the 
mills  in  the  Tonopah  and  Groldtield  districts  employ  the  "  slat 
method  "  of  support,  which  consists  of  sewing  the  canvas  walls 
of  the  cell  into  narrow  pockets  from  1.5  to  2  in.  wide,  and  into 
each  of  these  slipping  a  grooved  lath.  The  arrangement  is  low 
in  first-cost  and  very  satisfactory.  The  Moore  system  employs 


>    2ll_0 

IsH 

0 

I 

i 

f  > 

r 

1 

U   17 

/ 

<<  1 

i»M 

/ 

/ 

X.-.' 

*stt 

/ 

/ 

/ 

L- 

j 

f 

s 

3  LPT 

d 

rutf 

/ 

/ 

o  npL 

f 

j 

^  wit 

/ 

/ 

? 

I 

^ 

o  89LL 

/ 

H      7ft 

/ 

/ 

o   6rr 

f 

>1 

, 

A.  sand—  200  mesh 
5.  sand-150  mesh  +200  mesh 
C.  sand—  100  mesh  +150  mesh 
D.  sand—  80  mesh+100  nesh 
3  inch  bed 

5>     ft 

1 

/ 

?  ;tt~ 

1 

/ 

/ 

e    t 

X 

/ 

/ 

J 

w    1  1// 

X 

/ 

in    y= 

.-^ 

1   1   1 

=      0  10          20          30  10  50          60  70  80 

POUNDS  OF  WATER  PER  SQ.  FT.  PER  MINUTE 
FIG.  9. — FILTERING-KATES  FOR  FINE-SAND  BEDS. 

wooden  strips  slipped  into  narrow  pockets  in  the  canvas.  The 
Moore  system  also  makes  use  of  wire  netting  between  the  can- 
vas walls,  the  canvas  being  stitched  at  frequent  intervals 
through  the,  netting.  In  the  Oliver  filter,  wire  netting  over  a 
grooved  board  and  covered  with  8-oz.  burlap  supports  the 
canvas.  The  canvas  is  held  against  this  base  by  wire  wrapped 
around  the  canvas  at  0.5-in.  intervals.  In  both  the  Butters  and 
the  Moore  filters  wooden  dividing-strips  are  used  to  space  the 
filtering-surface  into  strips  1  ft.  wide.  Grooved  iron  plates  are 
used  in  the  filter-presses  and  in  the  Merrill  press. 

Durability  and  permeability  are  the  necessary  requirements 
of  a  filtering-cloth.  Canvas  duck,  army  weave,  No.  10,  answers 
both  of  these  requirements  for  suction-filters.  For  pressure- 

[14] 


SLIME-FILTRATION. 


853 


filters  this  canvas  is  too  light,  and  No.  6  gives  sufficient  dura- 
bility without  interfering  with  the  filtration  too  much.  On  ac- 
count of  the  wire  wrapping,  the  Oliver  filter  can  employ  a  lighter 
duck  (No.  12).  The  relative  permeability  of  different  weights 
of  canvas  is  a  difficult  matter  to  determine  experimentally.  It  is 
a  function  of  the  weave  of  the  cloth  and  the  character  of  the 
supporting  surface.  In  general,  the  lighter  the  weight  of  the 
duck  the  more  permeable  it  is.  Duck  may  be  obtained  in 
three  weaves  :  army  duck,  in  which  the  threads  of  warp  and 
woof  are  twisted;  double-fill,  in  which  the  warp  thread  is 
twisted  and  the  woof  threads  are  not  twisted;  single-fill,  in 
which  neither  the  warp  nor  the  woof  threads  are  twisted.  Of 


FIG.  10. — METHODS  OF  SUPPORTING  FILTER-CLOTH. 

the  three  weaves  the  army  duck  is  the  least  permeable,  while 
the  other  two  weaves  are  too  open  and  porous  to  be  of  much 
use  in  slime-filtration. 

Fig.  10  illustrates  several  methods  of  supporting  the  filtering- 
cloth.  In  method  1,  the  fibers  of  the  cloth  are  distended  and 
the  cloth  made  more  open  at  B,  while  at  A  the  fibers  are  flat- 
tened and  pressed  against  one  another,  with  the  result  of  re- 
ducing the  permeability  of  the  cloth  over  the  ridges.  The 
relative  proportion  of  ridge  to  groove  determines  the  decrease 
in  permeability  due  to  the  support.  No.  2  shows  the  wire  net- 
ting support,  and  with  this  the  rounded  wires  reduce  the  per- 
meability to  a  less  extent  than  the  flat  wooden  ridges.  In  No. 

[15] 


854 


SLIME-FILTRATION. 


3  the  narrow  wooden  strips  have  less  effect  than  the  close 
ridges  of  No.  1,  while  between  the  strips  the  cloth  is  stretched 
and  is  more  open.  With  No.  4  the  permeability  of  the  cloth 
is  not  generally  effected,  for  the  fibers  may  press  into  the  soft 
cocoa  matting.  In  No.  5  the  diamond-shaped  strips  give  the 
maximum  proportion  of  distended  canvas,  and  leave  the  fibers 
free  from  any  flattening  due  to  the  pressure. 

Fig.  11  shows  the  effect  upon  the  filtering-rate  of  the  type 
slime  for  three  filters.  Curve  L  is  for  No.  12  duck  on  a 
grooved  wooden  support  similar  to  No.  1,  Fig.  9 ;  41  per  cent, 
of  the  cloth  was  supported  and  59  per  cent,  unsupported. 
Curve  S  is  for  No.  12  duck  supported  on  diamond-shaped 


80 
ui;o 

2 

260 

cc 

Ul 

t" 

$40 

8 

o:30 
u 

Q. 

020 
°-10 
0 

Is 

1 

n 

1 

L.  No.12  Duck-ordinary  support  to  filter  cloth. 
S.  No.12  Duck-  open  support       to  filter  cloth. 
Dotted  curve.  No._10  Duck-  ordinary  support 
to  filter  cloth. 

1! 

1 

U 

\ 

\ 

^^X        c 

^L 

!s*--^ 

- 

^i-fcp 

^ 

i  

•^r*-^ 

?-L 

0.1       0.2      0.3  .     0.4       0.5      0.6       0.7       0.8       0.9       1.0       1.1      1.! 

THICKNESS  IN  INCHES 

FIG.  11. — FILTERING-KATE  OF  TYPE  SLIME  FOE  THREE  TYPES  OF  FILTERS. 

strips;  21  per  cent,  of  the  cloth  was  supported,  78  per  cent, 
unsupported.  The  dotted  curve  is  the  average  curve  for  the 
test-filter.  A  No.  10  duck  was  used  and  the  same  proportion 
of  support  given  the  cloth  as  for  curve  L.  The  heavier-weight 
duck  on  the  grooved  support  gives  a  higher  filtering-rate  than 
the  light  weight  on  either  the  grooved  or  the  more  open  sup- 
port. The  open  support  gives  a  higher  filtering-rate  for  the 
thinner  slime-cakes  and  lower  rates  for  the  thicker  cakes. 
This  anomalous  result  is  explained  by  the  fact  that  the  more 
permeable  the  filter  the  more  active  becomes  the  filtering- 
surface  for  a  given  pressure  and  the  more  compactly  the  cake 

[16] 


855 


SLIME-FILTRATION. 


is  built  up.  The  general  conclusion  is  that 
of  the  filter-cloth  is  a  matter  of  moderate  importance;  of 
greater  importance  with  the  thin-cake  suction-filters  than  with 
the  thick-cake  filters. 

No  experiments  could  be  made  on  the  comparative  durability 
of  filter-cloths.  From  data  submitted  in  Table  V.  it  appears 
that  suction-filters,  .supported  with  cocoa  matting,  have  the 
longest  life.  Close  stitching  is  of  importance  both  in  main- 
taining an  efficient  filter  and  in  prolonging  the  life  of  the 
filter-cloth.  The  first  Moore  filters  were  constructed  with  the 
canvas  supported  at  6-in.  intervals,  and  these  filters  failed  by 
tearing  and  weakening  generally  at  the  points  of  attachment. 
With  stitching  at  1-in.  intervals  the  wear  at  the  stitching- 
points  is  reduced  very  materially  in  the  Butters  filter.  With 
the  slat-filter  narrow  slats  allow  closer  stitching,  and  this  is  the 
tendency  in  construction. 

The  grooved  support  results  in  practically  a  clear  solution 
from  the  start  of  filtration  whether  a  No.  10  or  a  No.  12  duck  is 
used.  The  diamond-strip  support  and  No.  12  canvas  gave  a 
turbid  filtrate  for  the  first  minute  of  filtration. 

The  relation  between  numbered  duck  and  ounce  duck  is  as 
follows : 


Numbered  Duck. 

Ounce  Duck  29  In.  Wide 
and  36  In.  Long. 

Ounces  Per  Square  Yard. 

No.  12 
No.  10 

No.  8 
No.  6 

Ounces. 
8 
12 
15 
18 

10 
14—15 
18—19 
22—23 

Slime-  Cakes. 

A  slime-cake  is  built  up  of  a  succession  of  thin  layers  of 
slime.  Slices  taken  from  the  surface,  middle,  and  next  the 
canvas  showed  varying  percentages  of  water,  and  consequently 
a  variation  in  density  from  the  outer  surface  to  the  canvas. 
Fig.  12  represents  graphically  the  results  of  sectioning  dif- 
ferent slime-cakes.  B  shows  the  proportion  of  sand,  slime, 
and  water  for  a  cake  made  from  the  type  slime.  The  cake 
was  0.81  in.  thick,  and  the  vacuum  used  21  in.  The  outer 
0.25  in.  contained  41.4;  the  middle,  39.6;  and  the  portion 

[17] 


856  SLIME-FILTRATION. 

next  the  canvas,  33.3  per  cent,  of  water.  The  respective 
specific  gravities  are  1.575,  1.605,  and  1.715.  The  average 
water-content  is  38.1  per  cent.,  and  the  average  specific  gravity, 
1.632.  F  and  G  are  from  two  slime-cakes  in  which  the  re- 
spective proportions  of  sand  were  37  and  52  per  cent.  The 
composition  of  sections  of  these  cakes  is  given  in  Table  I. 

TABLE  I. —  Composition  of  Slime-Cake. 

CAKE  F. 

Surface  0.75  0.5  to          Next  Cany  as 

1  to  0.75  In.      to  0.5  In.         0.25  In.       0.25  to  0.0  In. 

Specific  gravity  of  cake,       .       1.755  1.805  1.822            1.835 

Per  cent,  of  water,       .        .  32.7  29.83  28.41  27.9 
Per  cent,  of  sand   in   dried 

cake,         ....  36.02  35.23  37.82  38.04 

Eatio  sand  to  slime,  1  to,     .       1.77  1.83  1.64              1.62 

Average  sp.  gr.,  1.804.          Average  percentage  of  water,  29.71. 
CAKE  G. 

Specific  gravity  of  cake,        .       1.776  1.814  1.853  1.841 

Per  cent,  of  water,       .         t  30.0  28.4  26.9  27.4 
Per  cent,  of  sand   in   dried 

cake,         ....  51.6  54.0  53.7  50.1 

Katio  sand  to  slime,  1  to,     .       1.08  1.10  1.17  1.22 

Average  sp.  gr.,  1.821.  Average  percentage  of  water,  28.4. 

Comparing  the  results  in  Table  I.  with  those  given  for  the 
type  slime,  a  variation  in  water-content  of  from  38.1  to  28.4 
per  cent,  and  in  density  of  from  1.632  to  1.821,  is  shown  for  a 
variation  in  sand  of  from  6  to  52  per  cent.  The  sand  in  these 
cakes  was  determined  on  the  basis  of  1-min.  settlement.  An 
increase  in  the  proportion  of  fine  sand  decreases  the  inter- 
stitial space,  but  does  not  decrease  the  permeability,  as  the 
curves  for  filtering-rates  show.  For  purposes  of  compari- 
son the  proportion  of  water  absorbed  by  fine  sands  is  shown 
in  Fig.  12.  Fine  sands  (quartz)  between  80-  and  100-mesh 
contain  26.4,  and  for  sands  passing  a  200-mesh  screen  and 
from  which  all  slime  was  elutriated,  28.1  per  cent,  of  water. 
The  volume  relationship  is  also  shown  for  both  fine  sands 
and  slime  in  Fig.  12.  The  former  shows  47.7  of  water  and  52.3 
per  cent,  of  solid ;  the  latter,  60  of  water  and  40  of  solid. 
These  figures  are  of  interest  in  that  they  show  the  compara- 
tively large  volume-proportion  of  water  in  the  slime-cakes. 

[18] 


SLIME-FILTRATION. 


857 


The  comparative  results  between  sand  and  slime  show  that  the 
percentage  of  water  is  no  indication  of  the  permeability  of  a 
porous  material. 

In  Fig.  12,  H  shows  the  results  for  the  Virginia  City  slime ; 
/,  for  the  type  slime  built  up  under  21.7-in.  vacuum  (36.2 
per  cent,  of  moisture  and  1.662  sp.  gr.) ;  J  shows  the  same 


Per  Cent 
100 

90 

80 


JSO 
.50 
JO 

j?0 

JO 
_0 

Perdeirf 

_9Q 

_JQ 

70 

.60 

J>Q 
JO 
_30 

10 
_0 


28.  \% 
Water 


-S0-H00mes,h      —  200  mesh  B  F  G 

Sand  Sand  Proportion  of  Water  by  Weight 


Water 


mm 


J  K 

Proportion  of  Water  by  Weight' 

I      i    i     i    r>v?i 

W.-it,-r         Slime          Sand 


35.125? 

Water 


Water 


47.7* 
Water 


Slime  San.l 

—  200  mebh 

Proportion  of  \Vater 
by  Volume 


FIG.  12. — SECTIONS  OF  SLIME-CAKES. 

slime  built  up  under  11.35-in.  vacuum  (36.75  per  cent,  of  mois- 
ture and  1.65  sp.  gr.);  K  shows  the  same  slime  built  up  under 
17-in.  vacuum  (35.4  per  cent,  of  moisture  and  1.675  sp.  gr.); 
L  shows  the  same  slime  built  up  under  21.8-in.  vacuum  (35.12 
per  cent,  of  moisture  and  1.683  sp.  gr.). 

The  data  of  Fig.  12  serve  also  to  show  the  effect  of  pressure 

[19] 


858  SLIME-FILTRATION. 

upon  the  specific  gravity  of  the  slime-cake.  To  these  may  be 
added  a  slime-cake  from  the  type  slime  which  was  built  up  on 
a  slat-supported  filter  of  No.  12  duck,  giving  with  21.5-in.  vac- 
uum a  moisture-content  of  33.6  per  cent,  and  specific  gravity  of 
1.71;  and  a  cake  built  up  under  30  Ib.  of  air-pressure,  giving 
28.1  per  cent,  of  moisture  and  1.805  sp.  gr.  With  the  excep- 
tion of  K,  there  is  an  increase  in  the  specific  gravity  with 
an  increase  of  pressure.  This  increase  in  the  density  of  the 
cake  means  a  decrease  in  the  permeability,  and  therefore  a 
decrease  in  filtering-rate.  A  slime-cake  may  be  likened  to  a 
number  of  layers  of  rubber  spheres.  Pressures  great  enough 
to  overcome  the  elasticity  of  the  spheres  would  have  the  effect 
of  squeezing  them  into  spheroids  and  reducing  the  intersti- 
tial space ;  still  greater  pressures  would  cause  the  spheroids  to 
encroach  upon  the  open  spaces  until  these  would  be  practically 
closed  and  the  interstitial  space  become  zero.  The  difficulties 
involved  in  using  high  pressures  upon  slime  carrying  a  mini- 
mum proportion  of  sand  are  apparent.  The  effect  of  the  pres- 
ence of  sand  would  be  the  same  as  if  angular  grains  were 
mixed  with  the  rubber  spheres.  Under  pressure  the  angular 
grains  would  press  against  one  another  and  prevent  any  great 
degree  of  pressure  coming  upon  the  spheres.  There  would  be 
a  comparatively  small  decrease  in  the  interstitial  space,  and 
therefore  little  or  no  reduction  in  permeability. 

An  interesting  phenomenon  was  noticed  in  transferring  slime- 
cakes.  The  lifting  of  the  cake  from  the  pulp  was  accompanied 
by  an  immediate  shrinkage  in  the  thickness  of  the  cake.  The 
reduction  in  thickness  amounted  to  from  10  to  12  per  cent. 
On  submerging  the  cake  in  the  pulp  the  original  thickness 
would  be  approximately  resumed.  The  effect  of  the  shrinkage 
is  to  increase  the  density  and  decrease  the  filtering-rate.  With 
sand-slime  cakes  no  noticeable  shrinkage  was  observed  until 
the  cake  approximated  a  thickness  of  1  in.,  and  in. all  cases 
this  shrinkage  was  less  than  that  of  the-slime  cake.  During  the 
building  up  of  a  slime-cake  there  is  a  progressive  densification, 
somewhat  slow  and  often  irregular,  which  accounts  for  the 
erratic  results  obtained  in  some  cases  with  the  filtering-rate 
experiments. 

The  progressive  densification  of  a  slime-cake  may  be  shown 
in  another  way.  Subjecting  a  thick  slime-cake  to  continued 

[20] 


SLIME-FILTRATION.  859 

pressure  when  immersed  in  water,  and  determining  the  filter- 
ing-rate at  several  successive  time-intervals,  gives  a  slow  drop 
in  the  filtering-rate. 

Increase  in  temperature  has  a  slight  effect  in  increasing  the 
density  of  the  cake,  but  the  experiments  on  this  point  were  on 
the  whole  somewhat  inconclusive. 

The  cracking  of  a  slime-cake  takes  place  under  two  condi- 
tions :  when  it  is  removed  from  the  pulp  and  allowed  to  remain 
in  the  air  under  full  pressure,  and  when  removed  from  a  pulp 
to  water  at  a  temperature  40°  or  50°  C.  higher  than  the  pulp. 
The  latter  condition  is  of  little  practical  importance.  The 
former  is  overcome  by  reducing  the  vacuum-pressure  to  just 
sufficient  to  hold  the  cake  upon  the  cloth.  Too  long  an  expo- 
sure even  at  this  pressure  will  cause  a  cake  to  crack.  Under  a 
vacuum-pressure  of  21  in.  a  1-in.  cake  will  break  down  in  from 
2  to  10  min.  Sand-slime  cakes  will  stand  a  longer  exposure 
than  slime-cakes.  The  cause  of  cracking  is  lateral  shrinkage, 
due  to  the  displacement  of  the  water  by  air  and  air-drying. 

In  building  up  cakes  with  vertically-suspended  filtering- 
cells  there  is  a  tendency  for  the  cake  to  build  up  thicker  at  the 
lower  end.  This  is  due  to  the  increase  of  filtering-pressure 
due  to  increased  hydrostatic  head,  and  also  to  the  thickening 
of  the  slime-pulp  in  the  lower  part  of  the  filter- vat.  Agitation 
will  prevent,  to  a  large  extent,  the  building  up  of  thick-ended 
cakes.  Where  the  proportion  of  sand  is  large  and  the  sand 
grains  are  coarse,  agitation  is  quite  necessary,  but  should  not 
be  too  vigorous,  as  otherwise  the  building  up  of  a  cake  would 
be  seriously  interfered  with  by  erosion.  With  fine  sands,  finer 
than  200-mesh  screen,  if  a  pulp-density  of  1.4  or  more  is  main- 
tained, little  or  no  trouble  is  experienced  by  the  sands  settling 
out.  Apparently  the  pulp,  in  the  experiments,  remained  quite 
homogeneous  for  intervals  of  longer  than  one  hour.  With  a 
greater  proportion  of  water  in  the  pulp  moderate  agitation  is 
necessary  in  most  cases. 

Slime-cakes  should  be  built  up  with  vacuum-pressures  as 
constant  as  possible,  and  should  be  kept  completely  submerged 
while  the  cake  is  forming.  The  temperature  of  the  pulp  and 
of  the  wash-water  should  be  the  same.  Transfers  from  pulp 
to  wash  should  be  made  as  rapidly  as  possible  and  under  re- 
duced pressures.  Filters  in  which  the  transfers  are  quickly 

[21] 


860  SLIME-FILTRATION. 

made,  and  with  the  minimum  of  exposure  of  the  cake  to  the 
air,  are  more  efficient  and  maintain  a  higher  filtering-rate  than 
those  in  which  long  time-intervals  are  required  for  the  neces- 
sary transfers. 

Building  Up  Slime-Cakes. 

Three  direct  factors  control  the  rate  of  and  the  total  time  re- 
quired for  building  up  the  cake :  the  thickness  of  the  cake,  the 
filtering-rate  of  the  slime,  and  the  proportion  of  water  in  the  pulp. 
Indirectly,  temperature,  viscosity  of  the  pulp,  intensity  of  the 
vacuum  used,  depth  of  submersion  of  the  cell,  agitation,  and 
the  physical  character  of  the  slime,  play  a  part  in  the  building 
up  of  the  cake.  The  effect  of  the  indirect  factors  is  summed 
up  in  the  filtering-rate. 

Practical  experience  has  placed  certain  well-defined  limits 
upon  the  thickness  of  the  slime-cake.  For  example,  the  Ridg- 
way  filter  utilizes  a  thickness  of  from  0.125  to  0.375  in.;  the 
Oliver,  from  0.25  to  0.5;  the  Butters  and  Moore,  from  0.75  to 
1.75;  the  Merrill,  from  1.75  to  2;  the  Kelly,  from  1  to  3, 
and  the  Burt  revolving-filter,  up  to  6  in.  With  vertically- sus- 
pended filters  the  thick  cakes  tend  to  tear  apart  and  drop 
during  the  transfers.  The  thickness  of  the  cake  also  deter- 
mines the  time  required  for  washing.  Thick  cakes  require 
relatively  a  much  longer  time  to  wash  than  the  thin,  on  account 
of  the  low  filtering-rates,  and  the  capacity  of  a  filtering- 
unit  may  be  very  greatly  cut  down.  With  slime-cakes  the 
lower  limits  mentioned  above  are  used;  with  sand-slime  cakes 
the  upper  limits  may  be  used. 

Other  things  being  equal,  the  less  solution  that  is  required 
to  be  drawn  through  a  filter  in  the  building  up  of  the  cake  the 
more  quickly  the  cake  will  be  secured,  and  consequently  it  is 
desirable  to  have  the  slime-pulp  as  ow  in  content  of  water  as 
possible.  There  is  a  practical  limit  to  the  thickening  of  the 
slime-pulp,  and  this  is  established  by  the  settling-power  of  the 
pulp  and  the  fluidity  of  the  settled  pulp.  The  settled  pulp 
must  be  handled  in  pipes  and  with  centrifugal  pumps,  and 
if  it  is  too  thick  it  becomes  impossible  to  do  this.  A  thick 
pulp  is  advantageous  where  much  sand  is  to  be  held  in  suspen- 
sion. In  Nevada,  with  quartzose  ores,  pulp-ratios  of  from  3  of 
solution  to  1  of  slime  down  to  1.5  of  solution  to  1  of  slime  are 
in  use.  The  average  pulp  in  use  is  2  of  solution  to  1  of  slime. 

[22] 


SLIME-FILTRATION. 


861 


Fig.  13  shows  the  relation  between  percentage  of  water  and 
specific  gravity  of  slime-pulp  and  cakes  for  a  slime  of  specific 
gravity  2.62.  Similar  curves  may  be  worked  out  for  slimes  of 
different  specific  gravity. 


1UU 

90 
80 

70 

cc 
jO 

<GO 
^50 

LJ 
O 

S40 
30 
20 
10 
0 

\ 

1:9 

\ 

\1:5 

\ 

:3 

- 

2 

\  \-.\y. 

1 

2 

g 

<a     o5 

CC 

.1 

\ 

E* 
\l:l 

? 

••o 

1 

N 

\ 

V 

l:^i 

"       ' 

^^> 

Sli 

ne    2.G2Sp;  O 

r. 

1.1 


1.3      :1.4        1.5       1.0       1.7       1.8       1.9       2.0 
SPECIFIC  GRAVITY 


FIG.  13.— RELATION  BETWEEN  PERCENTAGE  OF  MOISTURE  AND  SPECIFIC 

GRAVITY. 


70 


fc. 

H 


Per  cent. 
Sp.Gv.  Water 

Temp.Tacuum  Cake    in  cake 
?°C      21.3       1.642      38.L 
7.1        21.3 

10.75      15.3        1,653      36J 
9.6        21.2 


Sp.Gv. 
Pulp 
1.3 
1.305 
1.3 
1.4 


^ 


B 
^0 


0.1 


0.2      0.3       0.4      0.5      0.6       0.7      0.8       0.9       1.0      l 

THICKNESS  IN  INC.HES- 


l       l.»       !U8 


FIG.  14.—  KATIO  OF  BUILDING  UP  CAKES,  No.  10  CANVAS. 

Fig.  14  shows  the  rate  of  building  up  slime-cakes  on  ~No.  10 
canvas.  Curves  J5,  (7,  D,  E  are  for  the  type  slime;  curves  F 
and  G  are  for  the  type  slime  with  mixtures  of  sand  to  the 

[23] 


862 


SLIME-FILTRATION. 


amount  of  37  and  52  per  cent.,  respectively.  Curves  B  and  C 
show  a  rapid  consolidation  of  the  cake  at  0.25  in.  thick,  and 
then  a  gradual  thickening.  Curve  Z),  built  up  at  a  lower  pres- 
sure, shows  a  gradual  thickening  and  a  somewhat  greater  rate 
of  thickening  than  B  or  C.  Curve  E  shows  a  greater  rate  of 


70 


Per  cent.  Sp.Gv 
Temp.  Vacuum    Sp.gv:Cake  Moisture    Pulp 


60  L.  Slim* 
M.  Slime 
N.  Slime 
O.  Slime-sand  15* 
P.  Slime-sand  2i.3 
Q.  Slime-sand  33.2 


50 


1L2C 

23* 
35* 


0.1       0.2       0.3       0.4       0.5       0.6       0.7      0.8       0.9 
THICKNESS-INCHES 


3.0       1.1       1.2       1.3 


Fio.  15. — KATE  OF  BUILDING  UP  CAKES,  No.  12  DUCK. 


30 


TO 


61) 


50 


10 


Sp.jf  v.    Per  cent. 
Temp.  Vacuum.    Cake     Moisture,  Canvas  $p.  Gv.Pulp 
L.  Slime    10.5°C      21  in.        1.71         33,5          #12           1.40 
S.  Slime     11.7         21.5  in.        1.71          33.6           $12            1.41 
tf.  Slime      9.6        21.3            #10            1.41 

£2 

L 

^ 

/ 

' 

/ 

A 

j 

// 

^ 

^ 

/ 

^ 

7 
x 

X 

r  x 

/ 

ie-^ 

-^ 

X 

.x" 

S 

^X 

S 

—  *** 

^ 

J7,, 
=^S 

^L 

0.1      0.2 


0.4 


0.5       O.G       0.7       0.8 
THICKNESS  -INCHES 


0.9      1.0       1.1       1.2       1.3 


FIG.  16.  —  KATE  or  BUILDING  UP  SLIME-CAKES. 

thickening  on  account  of  the  greater  density  of  the  pulp.     The 
curves  for  the  sand-slime  cakes  show  a  gradual  thickening. 

Fig.  15  shows  the  rate  of  building  up  slime  and  sand-slime 
cakes  on  No.  12  duck  under  different  temperature-conditions. 
Curves  Z/,  M,  and  N  are  for  the  type  slime,  and  0,  P,  and  Q 

[24] 


SLIME-FILTRATION. 


863 


for  a  sand-slime  containing  52  per  cent,  of  sand.  All  three  slime- 
curves  show  greater  irregularities  than  the  sand-slime,  and  the 
sudden  consolidation  of  the  slime-cakes  between  0.3  and  0.45 
in.  in  thickness  is  characteristic.  Increase  of  temperature  in- 
creases the  rate  of  building  up  to  a  marked  extent. 

The  result  of  using  a  more  permeable  filter  is  illustrated  by 
comparing  Figs.  14  and  15.  The  significant  curves  are  shown 
in  Fig.  16.  Curve  L  is  the  type  slime  built  up  on  No.  12  duck 
supported  on  the  grooved  board;  curve  8  is  the  type  slime 
built  up  on  No.  12  canvas  and  diamond-strip  support.  The 
more  permeable  filters  show  faster  rates  for  the  thin  cakes  and 
slower  for  the  thick,  than  the  less  permeable  filter  with  No.  10 
duck.  The  diamond-strip-supported  filter  gives  a  faster  rate 
than  the  grooved  board. 


A.  17  inches  suction     )  1.4  specific  grayitf  of  pulp 

B.  11.35  inches  suctionV  •*•  \-'£ 
C.21.5.inches  suction  )  c.  12"C 

A.  caVe  contains  35.4  per  cent  water 
B.cake  contains  36.75  per  cent'  water 
C.«ake  contains  35.12  per  cent  water 


0.1       0.2      0.3      0.4       0.5       O.ti      0.7       0.8       0.9       1.0 

THICKNESS-INCHES 
FIG.  17.— BUILDING  UP  OF  CAKE  UNDER  YARYING  PRESSURES. 


Fig.  17  shows  the  effect  of  pressure  upon  the  rate  of  building 
up.  The  type  slime  and  No.  10  duck  were  used  in  these  ex- 
periments. Curve  B,  the  rate-curve  for  the  lowest  pressure,  is 
quite  uniform ;  curve  A  is  broken,  and  shows  three  consoli- 
dations ;  curve  C  shows  corresponding  but  not  such  prominent 
consolidations.  The  curves,  on  the  whole,  indicate  that  in- 
creased pressure,  other  things  being  equal,  will  increase  the 
rate  of  building  up. 

Fig.  18  shows  the  rate  of  building  up  a  slime-cake  from  a 
pulp  made  up  from  fire-clay.  The  very  slow  rate  and  the  four 

[25] 


864 


SLIME-FILTRATION. 


pronounced  consolidations  extending  over  comparatively  long 
time  intervals  are  of  interest. 

The  rate-curves  indicate  that  as  a  slime-cake  builds  up  a 
slow  consolidation  takes  place,  and  superimposed  upon  this  is 
an  irregular  and  faster  rate  of  consolidation.  The  irregular 
consolidation  is  characteristic  of  very  slimy  and  clayey  slimes, 
and  becomes  less  so  as  the  proportion  of  sand  is  increased. 
The  sand  diminishes  the  elastic  nature  of  the  slime-cake. 


120 

iro 

100 
90 
80 
70 

3 

^60 

z 
s 
50 

40 
30 
20 
10 

Sp.gv.  Slime  Pulp  1.356 
Ratio  1  dry  slime  to-2.15  water   • 
Cake  1.617  Sp.gv.  35.9  per  cent 
moisture 

i 

/ 

i 

/ 

i 

/ 

/ 

I 

f 

/ 

2 

/ 

/ 

^ 

/ 

0.1       0.2       0.3       0.4       0.5       0.6      O.T 

THICKNESS  OF  CAKE 
FIG.  18. — BUILDING  UP  CLAY  SLIME. 

Washing. 

A  slime-cake  retains  from  28  to  38  per  cent,  of  solution ;  the 
former  for  sand-slime  and  the  latter  for  slime-cakes.  Were  it 
not  for  osmosis,  simple  displacement  with  a  volume  of  water 
equal  to  that  retained  by  the  cake  would  be  sufficient  to  remove 
the  dissolved  salts  of  gold,  silver,  and  cyanide.  As  it  is,  the 
soluble  salts  diffuse  back  into  the  wash-water,  and  in  time  this 
builds  up  in  gold-  and  silver-values  to  such  an  extent  that  an 
appeciable  loss  results  when,  as  in  the  Butters  filter,  the  slime- 

[26] 


SLIME-FILTRATION.  865 

cake  is  flushed  out  with  the  residual  wash-water.  By  the  use 
of  two  separate  wash-solutions  this  difficulty  can  be  more  or 
less  overcome;  but  it  has  the  objection  that  an  additional 
pumping  of  solution  and  exposure  of  the  slime-cake  to  the  air 
are  necessitated.  With  the  Moore,  Oliver,  and  the  pressure- 
filters  generally,  no,  great  trouble  is  experienced  from  loss  of 
values  in  the  wash-solution,  for  in  each  case  only  that  wash- 
water  which  is  left  in  the  cake  after  air-displacement  takes 
place  is  discharged,  and  this  amounts  to  from  20  to  30  per  cent. 
In  practice  it  is  customary  to  use  barren  solution  or  wash-water 
in  amount  equal  to  from  one  to  three  times  the  amount  of  solu- 
tion retained  by  the  cake.  With  very  low-grade  solutions  the 
former,  and  with  high-grade  solutions  the  latter,  would  be  used. 
Twice  the  weight  of  the  solution  retained  by  the  cake  is  usu- 
ally sufficient  to  displace  the  values  retained  by  the  solution  in 
the  cake. 

The  thoroughness  of  washing  is  determined  largely  by  the 
expense.  When  the  value  of  the  gold,  silver,  and  cyanide  re- 
covered is  less  than  the  cost  of  recovery,  washing  stops. 

It  seems  to  me  that  with  the  thin-cake  filtering-appliances  a 
smaller  proportion  of  wash-water  would  be  required  than  for 
the  thiek-cake  filters,  for  the  reason  that  the  rate  of  filtration  is 
much  higher  with  the  former.  Strength  of  solution,  time,  and 
temperature,  are  the  controlling  factors  in  osmosis;  and  of 
these,  time  is  perhaps  the  most  important;  for  the  other  two 
would  be  the  same  in  either  case.  The  shortness  of  the  time 
required  for  washing  and  the  relatively  high  filtering-rate  in 
the  case  of  the  Oliver  filter  would  give  little  oportunity  for 
osmosis  to  interfere  with  the  washing. 

No  washing- experiments  were  made  with  suction-filters.  A 
few  experiments  were  made  with  a  pressure-filter  in  washing 
slime-cakes  containing  cyanide  solutions.  With  pressures  vary- 
ing from  20  to  30  lb.,  and  with  a  0.2-per  cent,  cyanide  solution, 
a  0.75-in.  cake  required  from  1.3  to  1.4  times  the  contained 
water  to  reduce  the  cyanide-content  to  0.02  per  cent. 


866 


SLIME-FILTRATION. 


Complete  Cycle  of  Filtration. 

Given  the  time  for  making  all  of  the  necessary  transfers,  the 
time  for  a  complete  cycle  of  operations  may  be  approximated 
by  using  the  filtering  and  building-up  rate-curves.  Two  such 
examples  have  been  worked  out  in  Figs.  19  and  20.  The 
former  shows  the  time  required  for  a  complete  cycle  for  the 
type  slime,  mixed  with  50  per  cent,  of  sand,  both  for  single 


Z  120 


f\ 

/ 

/ 

Slime  pulp  1.41  Sp.gv. 

£ 
i 

d 

^  = 

1  Perc-ehtage  ot-  Water  34 

/ 

/ 

— 

Sand- 
No.  12 
Cake 

Slime-pulp  UlS  Sp.gv. 
Dnek-80°C 

Assumed  Sp.gv.  1.82- 
Percentage  Water  28.i 

/ 

/Washing 

/ 

/• 

dot 

ble/dibpl. 

/ 

/ 

! 

* 

/ 

^ 

W;l 

,-k 

shin 

^dis 

y 

/ 

I/ 

/ 

1 

/ 

/ 

/ 

u 

*  c 

1 

/ 

/ 

t 

/W;ash 

£ 

1 

/ 

7 

/( 

oubte/Qispl 

1 

/ 

/ 

/ 

/Washing 

/   , 

f 

/ 

. 

/ 

/ 

./ 

single  di-jpk 

1    Jx4- 

/ 

/ 

/ 

/ 

Bu 

Idii 

-u 

p 

/ 

'  ^ 

^ 

SH 

// 

/ 

^ 

/ 

^ 

^ 

^ 

Build  ng-up 

// 

'/ 

. 

01  0.3  0.3  0.1  0.5  0.6  0.70.8  0.9  1.0   1.1  1.2  1.3         0.1  0.2  0.3  0.4  0.5  0.0  0.7  0.8  0.9  1.0  i.l   l.U  1.3 

Fio.  19. — CHANGES-TRANSFERS.  FIG.  20. — CHANGES-TRANSFERS. 

displacement  and  double  displacement ;  while  the  latter  shows 
the  time  required  for  the  type  slime.  The  24-hr,  capacity  per 
100  sq.  ft.  of  filtering-surface  may  be  calculated  from  the  dia- 
gram, and  the  specific  gravity  of  the  cake  and  the  percentage 
of  moisture  retained.  Table  II.  shows  the  results  of  such 
calculations. 


[38] 


SLIME-FILTRATION.  867 

TABLE  II. — Filtration :   Capacity  Per  100  Sq.  Ft.  of  Filtering- 
Surface  Per  24  Hr. 

SAND-SLIME  CAKE. 

0.25  In.  0.5  In.     0.75  In.       1  In.     1.25  In. 

Weight  of  cake,  pounds   (per 

cycle),         ....     236  473          710          947      1,183 

Weight  of  dry  slime,  pounds 

(per  cycle),          \         .         .170  340          510          681          851 

Number  of  cycles,  .         .         .       24  20.8        17.5         14.2        10.5 

Dry  slime  per  single  displace- 
ment, tons,          .        .        .        2.04  3.54        4.46        4.83        4.46 

Number  of  cycles  double  dis- 
placement,          .        .        .23.6  18.7        14.6        11.4          7.6 

Dry  slime,  tons,      .         .         .         2.0  3.2          3.7          3.88        3.2 

SLIME-CAKE. 

Weight  of  cake,  pounds  (per 
cycle), 221         442         663         885      1,016 

Weight  of  dry  slime,  pounds 

(per  cycle),        .        .        .146         292         438         585         731 

Number  of  cycles  single  dis- 
placement,"         .        .        .       22.8        15.1        11  8.4          6.7 

Dry  slime,  tons,     ...         1.6          2.2          2.4          2.45        2.38 

Number  of  cycles  double  dis- 
placement,         .        .        .21.8        14.1          8.7          6.5          5.1 

Dry  slime,  tons,      ...         1.5          2.04        1.9          1.9          1.8 

Table  II.  is  constructed  for  a  Butters  filter,  and  the  time  for 
all  the  transfers  is  taken  as  55  min.  These  data  clearly  show 
the  impracticability  of  using  thin  cakes  for  filters  of  this  type. 
Taking  power-costs  into  account,  it  is  advisable  to  have  as  few 
cycles  as  possible  in  filters  of  the  Butters  type,  and  this  would 
be  accomplished  by  building  up  thick  cakes. 

It  should  be  noted  that  the  capacities  calculated  are  higher 
for  the  slime-cake  than  pertain  in  practice,  for  the  reason  that 
a  thicker  slime-pulp  than  is  ordinarily  the  custom  was  used 
and  consequently  the  time  for  building  up  was  less. 

By  using  a  different  horizontal  axis  in  Figs.  19  and  20,  the 
time  required  for  a  complete  cycle  for  other  suction-filters  may 
be  read  off  and  calculations  made  for  capacity. 


[29] 


868 


SLIME-FILTRATION. 


C^l  rH  rH  O      O 

0      § 

.    0  O  O'  O      0 

c5    o 

8    a-    1    1    \    \      \ 

\ 

%   a           '  i 

t--  rH  LO   rH       O 
rH  rHO  0       O 

0     0 

OO  O  O     0 

0     0 

3-1 

.       §  CD  OOt^  LO     t- 

LO        LO 

CD      O  LO 

£ 

O       t^  rH 

00 

•S                           co 

-» 

1 

.       W  CO  Tf  0  LO      CD 

"     ^  c<i  TJ*  r>*  os    co* 

S  is  8    s 

§  rj<  LO  OS  <M  «<l     -^ 
6^COdLOr^      0^ 

3 

5              ** 

S 

^ 

8 

^ 

0)  <N  CO  CO  O      LO 

<N    ^LO                   : 



^ 

o 

•0 

^       ^   ^  JH  00  00      LO* 

to    ^w     c^ 

"    :    :    :    :      : 

1 

& 

LO 

! 

§  CO  CO  O  O     O 

^  1-7            <M 

g         CO  O  O  O     Tf« 

§ 

^>    °  <N  cq*  co  n*    rn* 

g           rH  rH  CO      rH 

^  ^^  s  s 

U    CO  LO  C^  rH  O      T}< 
n   CD  r-i                          CO 
O> 

£ 

Bl 

rH 

§        * 

i 
5 

s 

o 

^ 

S°,0oo 
'     S^^osco    <4 

0 

O       O  rH                           C^ 
g      rH     |                           CD 

^  ^H    ^rH 
O 

g        ^ 
§        §OJ050>O»O     CO 

g   2sS""a  S 

5  "• 

s 

hi 

V 

a 

H 

£ 

^J 

V 

r 

§             .       §  »OOSCO(M     c3 
rti                   ^                   ^^ 

LO                                         CO 

CO      ^  O    CD  O        ^ 

o     "3 

g        S         CO  Tf  CO  O      OS 

^               ^^  ^^  rH  O  O>      t^» 
w         J-    t^  rH                            (M 

s 

rH  rH 

K     ff 

PH 

0     * 

^ 

iif 

^ 

3 
$ 

g.|          COO  CO  CO     0 

(N      CO  O                       t^ 

CN       ^^ 

g           OS  rH  O       m      OS 

jv 

^  •>.             G^l  r—  ( 

LO  rH        co             <M 

s_   CD  C^                        ^* 

i 

H 

Eg 

S 

H 

9  S*o      co  LO  o  o 

Tf    co  o                 t^ 

H 

g°g        ^  CD  CO  00      CO 

CO      0^        S            ^ 

'    :   :   :    :     • 

•$ 

q 

_L  gjOQ        O  CO  CO  CO      CO 

s^^ 

4 

X  S 

P 

. 

3 

H 

oJ           OOO 

P,              rH  CO  (M  O      LO 

co      CO  O         S            S<1 

*        ^   °           rH                  CD 

g           C*  rH  0  '-0       CO 
^    Tl<  O  rH  r-4  O      90 

^J*             O  rH  rH  IO      00 

^-(        LO    r4           LO                  C<l 

hi    t^  rH                              C<1 

S 

G  <^>  G>          _A_ 

03            .    S    fcjn    60 

dT" 

§  ++         J      ' 

M     •        _S   Sr    r-r 

-*      fi'*l^  • 

rJ*      ' 

§r  . 

r  '    '-s§  'f^ 

^^ 

.S 

.3     "  S£pQ  ^  "o  aT 

'|1 

+  \  \  \  a  • 

^  ,/g  cs"  —  »  —  '§  a 

v  ^  /c<i  d 

T^       •    S     °          O»      r-,  ^J 

*"         *^  'S 

i    3  §'§ 

„  -|  °°  *0          g        M     " 

g   S    G   a,        V      2   ^ 

'3  '  1  g  S^ 

|«^8       ^r?^ 

*  g"  2   •  .§  o  g; 

•*•*  »S    S     "*J   C3 

o  •**  S    'S  «o  •: 

•^•^H       *5          ^     Q^   »j^    .^ 

|||   11 

g^    g|        ^      rH    g, 

0    *3      O  "rt          rj                    -J 

s  is  S    j^ 

Illllf 

[30] 


SLIME-FILTRATION.  869 

Data  from  Slime- Plants. 

Data  were  secured  from  a  number  of  slime-plants  in  Nevada 
through  the  courtesy  of  the  different  managers.  Table  III. 
shows  the  results  of  a  number  of  physical  analyses  of  slimes 
obtained  from  these  plants.  For  purposes  of  comparison  the 
type  slime,  the  sand-slime  mixture,  the  clay  slime,  and  the 
filtering-rates  are  included  in  the  table.  The  screen-analysis 
was  conducted  as  follows :  A  50-g.  sample  was  taken  and  mixed 
thoroughly  in  a  mortar  to  a  thin  pulp  and  then  poured  into  an 
800-cc.  beaker  and  sufficient  water  added  to  make  a  volume 
of  600  cc.,  or  approximately  a  ratio  of  1  of  slime  to  12  of  water. 
After  stirring,  beaker  No.  1  was  allowed  to  stand  2  min.  and 
the  contents  poured  into  a  beaker  of  the  same  size.  The  sand 
left  in  the  bottom  was  mixed  with  600  cc.  of  water;  allowed 
to  settle  2  min.;  contents  of  beaker  No.  2  were  then  poured 
into  beaker  No.  3  and  of  No.  1  into  beaker  No.  2 ;  after  stand- 
ing 2  min.  contents  of  No.  3  were  poured  into  No.  4,  of 
No.  2  into  No.  3,  and  of  No.  1  into  No.  2.  No.  1  was  filled 
again  and  all  beakers  allowed  to  stand  2  min.  Contents  of 
No.  4  were  poured  into  a  1.5-1.  beaker,  of  No.  3  into  No.  4, 
of  No.  2  into  No.  3,  and  of  No.  1  into  No.  2.  No.  1  was  left 
and  the  steps  repeated  until  all  four  beakers  were  empty  but 
for  the  sands  and  the  reject  in  two  1.5-1.  beakers.  The  sands 
were  washed  into  pans  and  dried  and  then  screen-analyzed 
through  100-,  150-  and  200-mesh  screens.  The  portion  of 
the  pulp  in  the  large  beakers  was  stirred  and  allowed  to  settle 
4  min.  and  poured  off;  the  sands  constituted  the  4-min.  por- 
tion. Stirring  and  standing  8  min.  gave  the  next  to  the  last 
portion,  and  the  last  portion  constituted  the  remainder.  The 
sands  obtained  by  this  method  were  clean  and  free  from  adher- 
ing grains  and  were  almost  all  silica.  The  size  of  the  grains 
in  the  various  portions  was  approximated  by  measurement  with 
a  microscope. 

The  type  and  the  clay  slimes  stand  out  conspicuously.  The 
type  slime  mixed  with  50  per  cent,  of  sand  and  the  mill-slimes 
compare  quite  closely.  Pulp  F  is  a  concentrate  treated  by 
agitation  and  pressure-filtration  in  a  Kelley  filter.  Practically 
all  of  the  mill-pulp  considered  as  slime  passes  a  100-mesh 
screen;  88  per  cent,  is  finer  than  a  No.  150  screen  and  79  per 
cent,  finer  than  a  No.  200  screen.  The  fine  portion  which 

[31] 


870 


SLIME-FILTRATION. 


takes  more  than  8  min.  to  settle  gives  an  average  of  27.5  per 
cent,  for  the  mill-pulps.  The  chemical  investigation  was  not 
completed,  but  the  results  of  partial  analyses  are  sufficiently  in- 
teresting to  include.  With  two  exceptions,  the  type  slime  and 
mill-slime  0,  the  calculated  percentage  of  aluminum  silicate 
corresponds  approximately  with  the  portion  of  the  pulp  taking 
longer  than  8  min.  to  settle.  Microscopical  examination  of 
the  coarser  portions  show  them  to  consist  almost  entirely  of 
silica.  The  results  on  the  type  slime  indicate  that  consider- 
able silica  remains  with  the  last  portion  of  the  slime.  Slime 
C  is  from  the  Goldfield  Consolidated  mill ;  as  is  well  known, 
part  of  the  alumina  is  present  as  alunite,  and  consequently  the 
calculation  of  all  of  the  alumina  to  aluminium  silicate  gives  a 
result  too  high.  The  results  of  the  chemical  analyses  indicate 
that  by  combining  physical  and  chemical  methods  an  approxi- 
mate separation  and  quantitative  determination  of  crystalline 
and  colloidal  material  may  be  effected. 

Table  IV.  gives  the  data  of  the  slime-plants  from  which  the 
mill-slimes  in  Table  III.  were  taken. 

TABLE  IV.— Details  of  Slime-Plants. 


A. 

J5. 

a 

D. 

E. 

F. 

G. 

Type  of  filter,  .     .     . 

Butters. 

Butters. 

Butters. 

Butters. 

Merrill. 

Kelley. 

Butters. 

Number  of  units, 

2 

2 

2 

2 

3 

1 

1 

Number  of  leaves  per 

unit,    

60 

95 

168 

72 

64 

10 

100 

Number  of  leaves, 

120 

190 

336 

100 

192 

100 

Area  of  leaf,  sq.  ft.,  . 

100 

91 

100 

92.6 

41.4 



100 

Total  filtering-area,  . 

12,000 

17,290 

33,600 

9.262 

7,980 

360 

10,000 

Tons  slime  per  24  hr., 

175 

250 

1,000 

150 

216 

100 

150 

Tons  slime  per  100  sq. 

ft.  per  24  hr.,      .     . 

1.4 

1.44 

2.97 

1.62 

2.72 

27.7 

1.5 

Slime-pulp   consist- 

ency, water  :  slime, 

3:1 

3:1 

1.5:1 

2:1 

2:1 

1.1 

2:1 

Filter-support,      .     . 

slats 

slats 

matting 

slats 

plate 

wire 
netting 

slats 

Thickness  of  cake,  in.  , 

1 

0.75-1 

1.25-1.75 

1.25 

1.75 

1.5-2 

1.5 

Moisture  in  cake,  p.c., 

38 

35 

35 

33 

12 

29 

Time  forming,  hr.,    . 

I 

0.5 

1-1.33 

1 

0.66 

0.03 

2 

Time  washing,  hr.,  . 

1 

1.0 

1.41-1.66 

1.25 

0.41 

0.20 

2 

Time  transfers,  hr.,  . 

0.75 

0.86 

0.59-0.51 

0.5 

3.01 



1.83 

Total  time-cycle,  hr., 

2.75 

2.33 

3-3.5 

2.75 

4.08 

0.5-0.75 

5.83 

Filtering-rate  per  100 

sq.  ft.  per  min.,      . 

11.6 

26 

8.3 

6.83 

30 

Filtering-rate  per 

min.  for  wash,  . 

10 

16.3 

Tons  of  solution  and 

wash  per  24  hr., 

848 

1,356 

1,000 

350-400 

328 
340 

•75 

O4 

Canvas  used,  oz., 

12 

12 

12 

12 

18 

12 

10 

Life  of  canvas,  months 

,      8 

6 

41est. 

18 

12 

0.5-1.5 

Frequency  of  acid- 

wash,  days,    .     .     . 

20 

21 

30 

21 

60 

15 

30 

Approximate  cost  per 

ton  slime  

$0.18 

$0.27 

$0.075 

$0.25 

$0.238 

$0.35 

$0.119 

• 

8-12  hrs. 

[32] 

SLIME-FILTRATION.  871 

Conclusions. 

Certain  practical  conclusions  may  be  drawn  from  the  experi- 
mental work.  These  have  been  in  part  stated,  but  may  not  be 
out  of  place  here,  together  with  certain  general  conclusions 
which  are  not  so  directly  shown  by  the  experimental  work. 
They  are: 

1.  The  proportion  of  clayey  material  in  ores  which  are  to  be 
subjected  to  "  all-sliming"  and  filtration  should  be  maintained 
at  a  minimum. 

2.  The  slime-pulp  should  be  as  free  as  possible  from  sands 
coarser  than  a  No.  150  screen,  and  as  large  a  proportion  of  the 
pulp  as  possible  should  consist  of  material  passing  a  No.  200 
screen. 

3.  The  slime-pulp  before  filtration  should  be  settled  to  as  thick 
a  consistency  as  possible  consistent  with  ready  handling  by 
pumps  and  in  pipes. 

4.  The  temperature  of  the  slime-pulp  should  be  maintained 
between  20°  and  30°  C.  or  higher. 

5.  The  temperature  of  the  wash-water  and  the  pulp  should 
be  the  same. 

6.  Vacuum-pressures  should  be  varied  until  the  proper  in- 
tensity for  the  given  slime  is  obtained. 

7.  Where  very  clayey  slime  is  to  be  filtered,  as  much  fine 
sand  (limited  as  stated  above)  should  be  crowded  into  the  pulp 
as  it  will  carry  without  undue  settling  and  clogging. 

8.  No.  10  canvas  supported  by  slats  gives  the  best  all-round 
service  for  the  thick  cake,  and  No.  12  canvas  on  wire  netting 
answers  the  requirements  for  the  thin-cake  filtering-machines. 

9.  With  slimes  containing  a  large  proportion  of  colloid  or 
clayey  material  pressures  greater  than  those  obtainable  with 
vacuum  apparatus  are  of  questionable  advantage. 

10.  With    slimes  containing   a   large  proportion  of  clayey 
material  the  vacuum-filters  should  be  used. 

11.  With   slimes   containing  a  small  proportion  of  clayey 
material  and  much  fine  sand  both  vacuum-filters  and  pressure- 
filters  could  be  used  with  perhaps  equally  good  results. 

12.  With  slimes  containing  much  coarse  and  fine  sand  the 
chamber-filters  with   air-agitation   and'  high  pressures  would 
perhaps  give  the  best  results. 

[33] 


872  SLIME-FILTRATION. 

13.  Of  the  vacuum-filters,  the  thin-cake  continuous  filters  are 
a  decided  improvement  over  the  thick-cake  filters. 

Acknowledgments. 

I  am  especially  indebted  to  many  of  the  students  of  the 
Mackay  School  of  Mines,  to  Jay  A.  Carpenter,  to  W.  S.  Palmer, 
and  to  many  of  the  superintendents  and  managers  of  milling- 
plants  in  Nevada  for  assistance  and  data.  In  closing  this 
paper  I  wish  to  express  regret  for  my  inability,  on  account  of 
the  pressure  of  other  duties,  to  carry  out  more  completely  closely- 
related  lines  suggested  by  the  experimental  work. 


[34] 


LOAN 


Renewed  books  are  subject 

• 

ISApr'tofo 


LD  2lA-60m-3,'65 
(F2336slO)476B 


Berkeley 


YC  68376 


